The src family kinase lck plays a critical role in T lymphocyte development and activation. Expression of lck is regulated by two promoters, termed distal and proximal promoters, in both mice and humans. Although the existence of these promoters was described 20 years ago, their function in developing and mature T cells has not been analyzed. To address this question, we have generated mutant BAC transgenes by deletion of either proximal or distal lck promoter. Studies of these mice have demonstrated selective expression of lck at preferential and different stages of development when driven by one or the other of its two promoters. Corresponding selective stages of T cell development and differentiation have been shown to be dependent upon promoter-specific lck expression. The proximal and distal promoters function at distinct stages of T cell development and have non-redundant roles in the developmental process. Studies have been initiated using CRISPR-Cas methodology to dissect the lck proximal and distal promoters and the interaction between them in determining locus access and differentiation stage-specific promoter activity. We investigated the function of tumor suppressor p53 in regulating proliferation and function of T lymphocytes. We have made the unexpected observation that antigen-specific proliferative responses of naive and memory CD4 T cells require the down-modulation of tumor suppressor p53. In the absence of TCR signal, IL-2 induces a sustained increase in p53 protein, which prevents proliferative responses despite strong signaling through the IL-2 receptor. In contrast, TCR signaling results in early termination of p53 protein expression by decreasing p53 mRNA as well as by strong transcriptional induction of the p53-regulating protein Mdm2. Down-modulation of p53 in response to antigen stimulation is in fact critical for antigen-specific T cell proliferation; and preventing p53 degradation by inhibiting Mdm2 results in sustained p53 protein levels and prevents antigen-specific T cell proliferation. These studies elucidate a critical role of p53 as a negative regulator of T cell proliferation. It is the termination of p53 elevation by TCR signaling that allows proliferative responses to occur, enforcing antigen specificity. Preliminary studies of p53 effect on antigen-inexperienced and memory T cell repertoire have strongly suggested that p53 affects the threshold of TCR signaling required for activation of unprimed T cells by specific antigen, and their subsequent differentiation into memory T cells. p53 thus appears to be an important regulator of antigen-specific T cell activation and in vivo response, proliferation, and differentiation.